1. Field of the Invention
The present invention relates to an inverter circuit that is suitably applicable to, for example, a display device using an organic EL (Electro Luminescence) element. The present invention also relates to a display device provided with the above-mentioned inverter circuit.
2. Description of the Related Art
In recent years, in the field of display devices that display images, a display device that uses, as a light emitting element for a pixel, an optical element of current-driven type whose light emission luminance changes according to the value of a flowing current, e.g. an organic EL element, has been developed, and its commercialization is proceeding. In contrast to a liquid crystal device and the like, the organic EL element is a self-luminous element. Therefore, in the display device using the organic EL element (organic EL display device), gradation of coloring is achieved by controlling the value of a current flowing in the organic EL element.
As a drive system in the organic EL display device, like a liquid crystal display, there are a simple (passive) matrix system and an active matrix system. The former is simple in structure, but has, for example, such a disadvantage that it is difficult to realize a large and high-resolution display device. Therefore, currently, development of the active matrix system is brisk. In this system, the current flowing in a light emitting element arranged for each pixel is controlled by a drive transistor.
In the above-mentioned drive transistor, there is a case in which a threshold voltage Vth or a mobility μ changes over time, or varies from pixel to pixel due to variations in production process. When the threshold voltage Vth or the mobility μ varies from pixel to pixel, the value of the current flowing in the drive transistor varies from pixel to pixel and therefore, even when the same voltage is applied to the gate of the drive transistor, the light emission luminance of the organic EL element varies and uniformity of a screen is impaired. Thus, there has been developed a display device in which a correction function to address a change in the threshold voltage Vth or the mobility μ is incorporated (see, for example, Japanese Unexamined Patent Application Publication No. 2008-083272).
A correction to address the change in the threshold voltage Vth or the mobility μ is performed by a pixel circuit provided for each pixel. As illustrated in, for example, FIG. 16, this pixel circuit includes: a drive transistor Tr100 that controls a current flowing in an organic EL element 111, a write transistor Tr200 that writes a voltage of a signal line DTL into the drive transistor Tr100, and a retention capacitor Cs, and therefore, the pixel circuit has a 2Tr1C circuit configuration. The drive transistor Tr100 and the write transistor Tr200 are each formed by, for example, an n-channel MOS Thin Film Transistor (TFT).
FIG. 15 illustrates an example of the waveform of a voltage applied to the pixel circuit and an example of a change in each of the gate voltage Vg and the source voltage Vs of the drive transistor Tr100. In Part (A) of FIG. 15, there is illustrated a state in which a signal voltage Vsig and an offset voltage Vofs are applied to the signal line DTL. In Part (B) of FIG. 15, there is illustrated a state in which a voltage Vdd for turning on the write transistor Tr200 and a voltage Vss for turning off the write transistor Tr200 are applied to a write line WSL. In Part (C) of FIG. 15, there is illustrated a state in which a high voltage VccH and a low voltage VccL are applied to a power-source line PSL. Further, in Part (D) and (E) of FIG. 15, there is illustrated a state in which the gate voltage Vg and the source voltage Vs of the drive transistor Tr100 change over time in response to the application of the voltages to the power-source line PSL, the signal line DTL and the write line WSL.
From FIG. 15, it is found that a WS pulse P is applied to the write line WSL twice within 1 H, a threshold correction is performed by the first WS pulse P, and a mobility correction and signal writing are performed by the second WS pulse P. In other words, in FIG. 15, the WS pulse P is used for not only the signal writing but also the threshold correction and the mobility correction of the drive transistor Tr100.